1. Field of the Invention
The present invention relates to a method of producing a semiconductor substrate and, more particularly, to a method of producing a semiconductor substrate which is formed on a transparent insulator substrate made of, for example, glass, and which is suitable for multi-functional and high-performance electronic devices or integrated circuits.
2. Description of the Related Art
Formation of a single-crystal silicon semiconductor layer on an insulator is widely known as the SOI (silicon on insulator) methods. Many studies have been made on the SOI methods because the substrate formed by these method achieves several advantages that a bulk Si substrate usually used to produce Si integrated circuits fails to achieve. The SOI methods and the substrate formed thereby have the following advantages:
1. Dielectric isolation is easy, so that high integration can be achieved.
2. The substrate has a strong resistance to radioactivity.
3. Floating capacity is reduced, so that the production process can be speeded up.
4. The well process can be omitted.
5. Latch-up can be prevented.
6. The substrate facilitates forming complete-depletion field-effect transistors when the substrate is formed into thin films. To achieve many advantages in device characteristics as listed above, many studies have been done on methods of forming the SOI structure for the past few decades. Such studies are concisely described in, for example, Special Issue, "Single-Crystal Silicon on Non-Single-Crystal Insulators" edited by G. W. Cullen, Journal of Crystal Growth, vol. 63, No. 3, pp 429-590 (1983).
The SOS (silicon on sapphire) method has been long known as one of the methods that are developed to such a high level as to facilitate forming substantially integrated circuits. In the SOS method, a silicon film is heteroepitaxially grown on a single-crystal sapphire substrate by using the CVD (chemical vapor deposit) method. Although this method is evaluated as a successfully developed SOI method, the lattice unconformity at the interface between the silicon film and the base sapphire substrate may cause substantial lattice defects, or a substantial amount of aluminium may leak from the sapphire substrate into the silicon film. Moreover, the sapphire substrate is costly, and it is difficult to form a large-area sapphire substrate. Therefore, the SOS method has not been widely applied.
Recently, many methods have been made to achieve use of silicon substrates, instead of sapphire substrates, for forming SOI structures. Such methods are roughly divided into the following three groups:
(1) A method comprising the steps of: oxidizing a surface of a single-crystal silicon substrate; removing a portion of the thus formed oxide film to partially reveal the silicon substrate; epitaxially growing a single-crystal silicon layer in the lateral direction on the SiO.sub.2 substrate by using the revealed portion as a seed.
(2) A method in which using a single-crystal silicon substrate as an active layer, an embedded layer of SiO.sub.2 is formed inside a lower portion thereof by a certain technique.
(3) A method comprising the steps of: sticking a single-crystal silicon substrate to an insulating substrate; and grinding or etching the silicon substrate so as to leave a single-crystal silicon layer having a desired thickness.
Basically , there are three different ways to achieve method (1). In one way (a vapor phase method), a single-crystal layer is epitaxially grown directly by the CVD technique. In another way (a solid phase method), a single-crystal silicon layer is epitaxially grown laterally with respect to the solid phase by heat-treating deposited amorphous silicon. In the third way (a liquid phase method), a single-crystal silicon layer is grown on an amorphous or polycrystal silicon layer either by melting recrystallization (beam annealing) which is performed by irradiating the silicon layer with a converged energy beam, such as electron beam or laser beam, or by zone melting recrystallization which is performed by scanning, in a belt-like course, the silicon layer with a rod-like heater. These methods have different advantages, but they have significant problems in controllability, productivity, and uniformity and quality of the products. The CVD (the vapor phase method) requires well-controllable grinding technique and sacrifice oxidation. The solid phase method fails to achieve satisfactory crystallization. The beam annealing of the liquid phase method has problems in time required for converged beam scanning, overlapping of the beam and controllability in, e.g., focus adjustment. The zone melting recrystallization is the most satisfactorily developed and has been tested to produce relatively large scale integrated circuits. However, it still fails to reduce the number of lattice defects, such as sub-boundaries, in the products to a required level, and therefore, the products of this method are not good enough to be used to produce minority-carrier devices.
To achieve method (2), many laboratories have been studying a method called SIMOX (separation by ion implanted oxygen), in which an SiO2 layer is formed inside a single-crystal silicon substrate by injecting oxygen ions. SIMOX is well matched with the silicon process and regarded as the most satisfactorily developed technique in method (2). However, 10.sup.18 oxygen ions or more per square centimeter must be injected in order to form an SiO.sub.2 layer. Such oxygen injection requires a long time, resulting in low productivity and increasing the cost of wafers. Moreover, a substantial number of lattice defects are still produced in the products, and therefore, the products are not good enough to be used to produce minority-carrier devices on an industrial scale. Another method is known in which SOI structure is formed by a technique of dielectric isolation which is performed by oxidizing a porous silicon. In this method, n-type silicon layer patches are formed on a surface of a p-type single-crystal silicon substrate by either injecting proton ions (Imai, et al., J. Crystal Growth vol. 63, 547 (1983)) or epitaxial growth and patterning, and many pores are formed selectively in the p-type silicon substrate so that porous p-type silicon substrate surrounds the n-type silicon layer patches, by anodization in HF solution, and then, the n-type silicon layer patches are dielectric-isolated by accelerated oxidation. Because the areas of revealed non-porous p-type single-crystal silicon are formed before the device processes, freedom in designing a device is often limited.
Basically, there are two methods to achieve method (3). In one method, a silicon substrate is used as a supporting substrate (in this case, the surface on one side of the silicon substrate is oxide). In the other method, an insulating substrate made of material other than silicon is used as a supporting substrate. In both methods, after the supporting substrate is stuck to another silicon substrate, heat treatment at a temperature about 1,000.degree. C. is performed in order to enhance the bonding between the two substrates. No matter what material may be used for the supporting substrate, the most crucial process is to make the silicon substrate into a thin film. Generally, a silicon substrate having a thickness of several hundred micrometers must be ground or etched to a uniform thickness of several micrometers, in some cases, 1 .mu.m or less. There are technical difficulties in achieving good controllability of the process and substantial uniformity of the pieces. In the method employing an non-silicon insulating supporting substrate, problems are often caused by the heat treatment at 1,000.degree. C. Because the coefficients of thermal expansion of the two substrates are different, the silicon substrate and/or the supporting substrate stuck to each other may warp, break or come off from each other (in some cases, the two substrates do not stick to each other from the beginning). Materials having coefficients of thermal expansion close to that of silicon have been tested. However, no known materials have heat resistances for the temperatures of the heat treatment for enhancing the bonding and the device-forming process.
As described above, a method of producing with good productivity SOI substrates which can be suitably used to produce high-performance electronic devices has not been developed. Particularly, for forming an SOI structure on a transparent substrate so that the substrate acquires functions, methods (1) and (2) are naturally inappropriate. Even the method 3 is not very suitable for that purpose. In most cases where the method 3 is employed, a supporting silicon substrate and a silicon substrate are stuck to each other by using a thermally oxidized film therebetween. It is very difficult to form an SOI by grinding the silicon substrate stuck onto the transparent supporting substrate having a coefficient of thermal expansion different from that of silicon.